The present invention relates to a power supply apparatus suitable for use in starting and driving lamps such as discharge lamps.
An example of discharge lamps is a xenon lamp. As shown in FIG. 1, a xenon lamp includes a glass tube 2 in which an anode electrode 4 and a cathode electrode 6 are spaced by several millimeters. The tube 2 is filled with xenon gas at several atm. An arc discharge is produced between the tip end 4a of the anode electrode 4 and the tip end 6a of the cathode electrode 6 when constant current flows between the anode electrode 4 and the cathode electrode 6, and the lamp starts to emit light at voltage and current at a stable operating point C on a xenon lamp current-voltage characteristic curve shown in FIG. 2.
As the end of the life of the xenon lamp approaches, the amount of consumption of the anode electrode 4 and the cathode electrode 6 increases, and the pressure within the glass tube 2 reduces, resulting in increase of impedance of the xenon lamp. This, in turn, causes the voltage between the anode electrode 4 and the cathode electrode 6 to increase, so that the stable operating point moves from the point C to a point D. This means the power consumption of the xenon lamp increases, and the heat generated also increases, resulting in melting of the anode electrode 4 and the cathode electrode 6. In the worst case, the xenon lamp may burst.
Japanese Examined Patent Publication (KOKOKU) No. SHO 59-37556 B2, published on Sep. 10, 1984, the applicant being the assignee of the present application, discloses an apparatus in which current flowing through a xenon lamp is reduced when the voltage applied to the lamp increases above a predetermine value, whereby increase of power consumed by the lamp is suppressed. The technique to reduce the current in the xenon lamp shown in this Japanese publication is to provide constant-power control, not constant-current control, when the output voltage exceeds a reference voltage, so that the stable operating point moves to a point E on the characteristic curve shown in FIG. 2.
It may occur that the xenon lamp having its life nearing to its end is energized to emit light by causing an arc to be generated between the tip end 4a of the anode electrode 4 and the tip end 6a of the cathode electrode 6 and, then, deenergized to stop emitting light by interrupting the supply of power to the lamp. In such a case, it may sometimes occur that when the lamp is re-energized after it is cooled down, an arc is generated not between the tip end 4a of the anode electrode 4 and the tip end 6a of the cathode electrode 6, but between other parts of the electrodes. For example, an arc may tend to be generated between a side 4b of the anode electrode 4 and the tip end 6a of the cathode electrode 6 as shown in FIG. 1. Since the distance between the side 4b and the tip end 6a is larger than the distance between the tip ends 4a and 6a, the impedance of the xenon lamp increases. According to the invention disclosed in the above-cited Japanese publication, because an increased impedance causes the voltage between the anode electrode 4 and the cathode electrode 6 to exceed a predetermined voltage, constant-power controlled power is applied to the lamp. The applied power may be insufficient to generate an arc discharge, so that the xenon lamp may not be ignited. In this case, if the xenon lamp is ignited by some means, the arc discharge between, for example, the side 4b and the tip end 6a gradually moves to the space between the tip ends 4a and 6a and is stabilized.
An object of the present invention is to provide a power supply apparatus which not only can ignite a lamp having a life near to its end, but also can sustain a stable operation of the lamp once ignited, by providing constant-power control.
A power supply apparatus for lamps according to the present invention includes a DC power supply. The DC power supply may be provided by converting commercial AC power into DC power. A DC-to-DC converter is provided for converting a DC signal from the DC power supply to have a predetermined value for application to a lamp. The DC-to-DC converter is a controllable. converter which can be controlled to provide output current or power having a predetermined value in response to a input control signal described later. A DC current detector detects a DC current supplied from the DC-to-DC converter to the lamp and develops a DC current representative signal representing the detected DC current. A DC power detector detects DC power supplied from the DC-to-DC converter to the lamp and develops a DC power representative signal representing the detected DC power. An error signal generating unit is provided, which generates a DC current error signal representing the difference between the DC current representative signal and a predetermined DC current reference signal, and a DC power error signal representing the difference between the DC power representative signal and a predetermined DC power reference signal. The DC current error signal is outputted during a predetermined time period from the start of the operation of the power supply apparatus. A control unit controls the DC-to-DC converter in such a manner that the signals from the error signal generating unit become zero.
By appropriately determining the DC current reference signal, a large current can be supplied to a lamp when the power supply apparatus starts operating, and, therefore, the lamp is turned on without fail even when the life of the lamp is near its end. In case of old lamp, once it is turned on, it can be constant-power controlled, and, therefore, the current supplied to the lamp can be decreased.
The error signal generating unit may include level adjusting means which makes the DC current error signal larger than the DC power error signal when the impedance of the lamp is smaller than a predetermined value, and makes the DC current error signal smaller than the DC power error signal when the impedance of the lamp is larger than the predetermined value. In this case, the error signal generating unit includes further selecting means for selecting a larger one of the DC current error and DC power error signals, and an interrupting circuit for interrupting the generation of the DC power error signal for a predetermined time period starting from the start of supplying the DC signal to the lamp.
With this arrangement, when current starts to be supplied to the lamp, the interrupting circuit prevents the DC power error signal from being generated. Accordingly, the selecting means selects the DC current error signal and applies it to the control unit. The control unit controls the DC-to-DC converter in such a manner as to supply a constant current to the lamp. Thus, for a predetermined time period following the energization of the lamp, a large current is supplied to the lamp, whereby the lamp is turned on without fail even when the lamp has a high impedance. Once the lamp is turned on, the selecting means selects the power error signal when the lamp has a life near to the end, and applies it to the control unit. Accordingly, the power supply apparatus is constant-power controlled so that the current supplied to the lamp is decreased. On the other hand, if the lamp is relatively new one, the selecting means selects and applies the current error signal to the control unit, so that the constant-current control is performed.
The interrupting circuit may include a charged circuit charged by the current representative signal, and a clamp circuit for clamping the power representative signal when the output signal of the charged circuit is smaller than a predetermined interruption reference signal.
With this arrangement, the current representative signal supplied to the charged circuit is substantially constant because the power supply apparatus is constant-current controlled. The charging of the charged circuit with substantially constant-value signal enables precise measurement of the predetermined time during which the generation of the DC power error signal is to be interrupted.
The power detector may include a voltage detector developing a DC voltage representative signal representing the DC voltage applied to the lamp, a level adjuster for adjusting the level of the DC current representative signal, and a multiplier for multiplying the output signal of the level adjuster and the DC voltage representative signal together.
Appropriate adjustment of the level of the DC current representative signal through the level adjuster makes the DC current error signal larger than the DC power error signal when the impedance of the lamp is smaller than the predetermined value and makes the DC current error signal smaller than the DC power error signal when the impedance of the lamp is larger than the predetermined value. Thus, the selecting means can be of a simple configuration which only selects a larger one of the DC current and power error signals. Also, the current detector can be used as part of the power detector, which can simplify the circuit configuration of the power detector.
In addition to the above-described arrangement, the error signal generating unit may include a constant-power control error amplifier which is supplied with the output signal of the multiplier and the DC power reference signal and develops the DC power error signal, and a constant-current control error amplifier which is supplied with the current representative signal from the current detector and the DC current reference signal and develops the DC current error signal. In this case, the selecting means includes a first input terminal at which the DC power error signal from the constant-power control error amplifier is applied, a second input terminal at which the DC current error signal from the constant-current control error amplifier is applied, and an output terminal connected to the control unit. The selecting means includes further a first switching device which is connected between the first input terminal and the output terminal and which is rendered conductive when the signal level at the first input terminal is higher than the signal level at the output terminal. A second switching device of the selecting means is connected between the second input terminal and the output terminal and is rendered conductive when the signal level at the second input terminal is higher than the signal level at the output terminal.
Since the power detector includes the level adjuster, the error signal generating unit can be formed of two error amplifiers, and, in addition, the selecting means can be of a simple configuration using two switching devices which can switch without resort to any special control signals therefor.
The error signal generating unit may have a first DC current reference signal and a second DC current reference signal smaller than the first DC current reference signal. In this case, the error signal generating unit operates to generate the DC current error signal based on the first DC current reference signal and the DC current representative signal for the predetermined time period starting from the start of supplying the DC signal to the lamp. When the predetermined time period lapses, the error signal generating unit generates the DC current error signal based on the second DC current reference signal and the DC current representative signal.
With this arrangement, the current supplied to the lamp when the lamp is started is large because it is prepared based on the first DC current reference signal, and, therefore, even an old lamp of which the life is nearing the end can be started without fail.
The error signal generating unit may include a constant-current control error amplifier to which the DC current representative signal is applied, and a selecting circuit. The selecting circuit receives first and second DC current reference signals and operates in accordance with the output signal from the interrupting circuit to apply the first DC current reference signal to the constant-current control error amplifier during the predetermined time period starting from the start of supplying the DC signal to the lamp and, thereafter, to apply the second DC current reference signal to the constant-current control error amplifier.
This circuit configuration is simple because of the switching of the reference signals to be supplied to a single constant-current control error amplifier.